In complex integrated circuits (ICs) there may arise the need to know whether a particular block of the circuit has correctly started up or to check in which operation condition the block is currently working. For example, in some cases it may be useful to know whether or nota linear amplifier is in a condition where it has enough DC gain.
Namely, an IC in general is able to perform several different functions, and the choice of how the IC has to react to external stimuli may depend on surrounding conditions of the IC. In case of transient conditions, it might occur that the IC is not in the appropriate operation condition to react to an external stimulus. In such cases, it is important to have univocal knowledge of whether or not the IC works in the correct operating condition.
One way to detect whether the IC works in the correct operating condition is to check whether or not sensitive Metal-Oxide-Semiconductor (MOS) transistors of the IC work in their saturation region. FIG. 11 illustrates a circuit 1100 for detecting whether an n-channel MOS (NMOS) transistor 10 is in saturation. In general, a MOS transistor is in saturation when the difference between the voltage at its drain terminal, VD, and the voltage at its source terminal, VS, is larger than the difference between the gate-source voltage of the MOS transistor, VGS, and the threshold voltage of the MOS transistor, VTH, i.e. when VD−VS>VGS−VTH. The circuit 1100 comprises a comparator 90 with an intrinsic offset that matches the voltage VGS−VTH of the MOS transistor. The intrinsic offset of the comparator 90 may be generated by means of a voltage source 95 that outputs a voltage of VG−VTH. If the saturation condition is satisfied, the comparator 90 will flag this by outputting a logical “1”, otherwise the comparator will output a logical “0”.
However, this approach has the disadvantage that the intrinsic offset of the comparator 90 has to match the variation of the voltage VGS−VTH over corners of the operation range of the MOS transistor and also has to account for temperature variation.
Further approaches of detecting an operation state of a MOS transistor may have the disadvantage that they may impact operation of the MOS transistor in linear region.
Thus, there is a need for an improved circuit for determining whether a transistor device is in a predetermined operation mode, and for an improved method of determining whether a transistor device is in a predetermined operation mode.